To meet users' demands, most earphones configured for present electronic devices have a noise reduction function and are capable of canceling external noise, so that people are “isolated from the world” after putting on earphones and shall not hear external noise. However, this manner results in that a user often cannot give timely feedback on an external situation. For example, if a user is listening to music with earphones while bicycling on a road, the user would not hear sounds of horns of nearby vehicles, which easily leads to an accident. Similarly, when a user is listening to music while exercising, the user would not hear greetings from others. There are still a lot of similar situations, which cause unnecessary troubles for users.